5.5 Aerosol Effects in Future Scenarios

5.5.1 Introduction

Aerosol concentrations and forcing will change in the future, both as a result
of changing emissions and as a result of changing climate. The uncertainties
associated with our knowledge of the present day distribution of aerosols noted
in previous sections will carry over into uncertainties in analysis of future
scenarios. Nonetheless, models are the best available tool for making an assessment
of what changes might follow. To estimate these future changes, we specified
a set of emissions for the IPCC model intercomparison workshop (Section
5.4.1) based on the draft scenarios developed for the IPCC Special Report
on Emissions Scenarios (SRES) (Nakic´enovic´ et al., 2000). The
results from the workshop form the basis of the future aerosol forcing reported
in Chapter 6 and contribute to the climate change scenarios
reported in Chapter 9.

Separate estimates for the amount of biomass-burning activity were not available
for the SRES scenarios (though growth in biomass burning was included in the
SRES analysis). Also, estimates of emissions of organic carbon and black carbon
aerosols from fossil fuels and industrial activity were not available. Therefore,
these were constructed using the ratio of source strengths for CO in 2030 and
2100 to that in 2000, respectively. This ratio was then used to scale the emissions
for organic carbon and black carbon from fossil fuel and biomass burning. Because
the scenarios do not provide a breakdown of emissions for CO by source category,
this scaling implicitly assumes that as a given region develops, the ratio of
emissions of CO by biomass burning and by fossil fuel burning remains roughly
constant. We note that our projected carbon particle emissions may be too large
if countries choose to target particle emissions for reduction to a greater
extent than they target emissions of CO.

In addition to the emissions of carbon particles, we constructed emissions
for NH3 in order to examine possible changes in the emissions of
NH3 and HNO3 to aerosol abundance and forcing. Only the
A2 scenario in 2100 was considered. For this simulation, the growth in anthropogenic
NH3 emissions was assumed to follow the growth in anthropogenic N2O
emissions. Anthropogenic emissions grew from 46.9 TgN/yr in 2000 to 111.5 TgN/yr
in 2100. The anthropogenic NOx emissions were 39.5 TgN/yr in 2000 and grew to
109.7 TgN/yr in 2100.

The emissions for 2030 and 2100 from the draft A2 and B1 scenarios that were
considered in the IPCC workshop are shown in Table 5.7
(see the Appendix to this volume for the final SRES emissions). As noted there,
SO2 emissions in 2030 are about a factor of 1.6 higher than those
in 2000 in the draft A2 scenario, but decrease thereafter to global average
levels that are less than the present-day estimates in 2100. Carbon aerosol
emissions grow by a factor of 1.3 in 2030 in the A2 scenario and continue to
grow to 2100 by an additional factor of 1.8. In the B1 scenario, both SO2
and carbon aerosol emissions are controlled by 2100, falling by 60 and 3%, respectively,
compared to 2000. For comparison, the table also shows emissions for the IS92a
scenario, with carbon aerosol emissions constructed as above for the SRES scenarios.
In the IS92a scenario, growth continues throughout the time period for both
SO2 and carbon emissions: SO2 and CO emissions are a factor
of 1.8 and 1.6 larger in 2100, respectively, than the same emissions in 2000.